An experimental procedure to obtain the equilibrium contact angle from the Wilhelmy method

The theory of contact angles has been developed using the concept of the Young contact angle, an equilibrium quantity related to perfectly smooth and homogeneous surfaces; on real surfaces it is commonly accepted that one can obtain "metastable" equilibrium states, in l,Inch the shape of the meniscus along the triple line is not fully equivalent to the "equilibrium" meniscus. In the present paper, a new method is presented to obtain an equilibrium contact angle with a simple modification of the Wilhelmy microbalance. The supply of acoustic energy, to the liquid at a frequency which induces the resonant formation of capillary waves, followed by the reduction of the amplitude Of the vibration, allows the meniscus to move from the metastable advancing or receding shape to the stable equilibrium one.